Calculate Zener Diode Series Resistor

Calculate series resistor for a Zener diode with constant load

Zener Diode Series Resistor Calculator

Voltage Regulation

Calculation for constant load. The series resistor must limit and stabilize the total current (load + Zener diode).

Select Load Input

Load current I_L input

Load resistance R_L input

Input voltage U_in

Output voltage U_Z

Load current I_L

Load resistance R_L

Zener minimum current I_Z,min

Decimal places

Standard Zener Diode Values

E12 Series (±5%):

2.7V 3.3V 3.9V 4.7V 5.1V 5.6V 6.2V 6.8V 7.5V 8.2V 9.1V 10V 12V 15V 18V 22V

Results

Series resistor R_s:

Resistor power dissipation:

Diode power dissipation:

Total current:

Zener Diode Voltage Regulator

Circuit diagram: Zener diode with series resistor

Operating Principle

Zener diode stabilizes the output voltage
Constant total current through series resistor
Current division between load and Zener diode
Minimum current through Zener diode required

Important Notes

Constant load: This calculator is for constant load

Minimum current: 10% of load current or min. 5-10mA

Power: Zener diode must be adequately rated

Variable load: Use different calculator for varying load

Basic Formulas

Total current: \[I_{total} = I_L + I_{Z,min}\]

Series resistor: \[R_s = \frac{U_{in} - U_Z}{I_{total}}\]

Practical Calculation Examples

Example 1: 5V Voltage Regulator

Given: U_in = 12V, U_Z = 5.1V, I_L = 100mA

Step-by-Step Calculation

1. Set Zener minimum current: \[I_{Z,min} = 10\% \times I_L = 0.1 \times 100mA = 10mA\]

2. Calculate total current: \[I_{total} = I_L + I_{Z,min} = 100mA + 10mA = 110mA\]

3. Calculate series resistor: \[R_s = \frac{U_{in} - U_Z}{I_{total}} = \frac{12V - 5.1V}{110mA} = \frac{6.9V}{0.11A} = 62.7Ω\]

4. Resistor power dissipation: \[P_{Rs} = (U_{in} - U_Z) \times I_{total} = 6.9V \times 110mA = 759mW\]

5. Zener diode power dissipation: \[P_Z = U_Z \times I_{Z,min} = 5.1V \times 10mA = 51mW\]

Result: R_s = 62.7Ω (E12: 68Ω), P_Rs ≥ 1W, P_Z ≥ 0.5W
Efficiency: η = 5.1V/12V = 42.5%

Example 2: 3.3V for Microcontroller

Given: U_in = 5V, U_Z = 3.3V, I_L = 50mA

Step-by-Step Calculation

1. Set Zener minimum current: \[I_{Z,min} = 10\% \times I_L = 0.1 \times 50mA = 5mA\]

2. Calculate total current: \[I_{total} = I_L + I_{Z,min} = 50mA + 5mA = 55mA\]

3. Calculate series resistor: \[R_s = \frac{U_{in} - U_Z}{I_{total}} = \frac{5V - 3.3V}{55mA} = \frac{1.7V}{0.055A} = 30.9Ω\]

4. Resistor power dissipation: \[P_{Rs} = (U_{in} - U_Z) \times I_{total} = 1.7V \times 55mA = 93.5mW\]

5. Zener diode power dissipation: \[P_Z = U_Z \times I_{Z,min} = 3.3V \times 5mA = 16.5mW\]

Result: R_s = 30.9Ω (E12: 33Ω), P_Rs ≥ 1/4W, P_Z ≥ 0.5W
Efficiency: η = 3.3V/5V = 66%

Example 3: 15V Reference Voltage (Precision Application)

Given: U_in = 24V, U_Z = 15V, I_L = 5mA (OpAmp reference)

Detailed Analysis for Precision Application

1. Zener minimum current (higher for stability): \[I_{Z,min} = 2 \times I_L = 2 \times 5mA = 10mA\] Higher current for better temperature stability

2. Calculate total current: \[I_{total} = I_L + I_{Z,min} = 5mA + 10mA = 15mA\]

3. Calculate series resistor: \[R_s = \frac{U_{in} - U_Z}{I_{total}} = \frac{24V - 15V}{15mA} = \frac{9V}{0.015A} = 600Ω\]

4. Resistor power dissipation: \[P_{Rs} = (U_{in} - U_Z) \times I_{total} = 9V \times 15mA = 135mW\]

5. Zener diode power dissipation: \[P_Z = U_Z \times I_{Z,min} = 15V \times 10mA = 150mW\]

6. Temperature coefficient: At 15V: TC ≈ +2mV/°C
For precision: temperature compensation needed

Result: R_s = 600Ω (E12: 560Ω), P_Rs ≥ 1/4W, P_Z ≥ 0.5W
Precision: For highest accuracy use reference IC (LM4040, etc.)

Zener Diode Component Selection

Voltage tolerance: ±5% (E12) or ±2% (E24)

Power: 0.5W (standard) to 5W (power types)

Temperature coefficient: <6V: negative, >6V: positive

Optimum: 5.6V to 6.8V (lowest TC)

Design Guidelines

Minimum current: 10% of I_L or min. 5mA

Resistor: E12 series, min. 2× calculated power

Efficiency: ~40-70% (depends on voltage ratio)

Alternative: LDO regulator for higher efficiency

Theory and Applications of Zener Diode Voltage Regulation

Operating Principle

The Zener diode voltage regulator uses the constant breakdown voltage of a Zener diode for voltage stabilization. The series resistor limits the total current, while the Zener diode conducts excess current and keeps the output voltage constant.

Current Distribution

Constant total current: Same current always flows through the series resistor
Variable current distribution: I_total = I_L + I_Z
Stabilization: Zener diode compensates for load current variations
Minimum current: Zener diode needs minimum current for stability

Mathematical Relationships

Total current:

\[I_{total} = I_L + I_{Z,min}\]

Series resistor:

\[R_s = \frac{U_{in} - U_Z}{I_{total}}\]

Resistor power dissipation:

\[P_{Rs} = (U_{in} - U_Z) \times I_{total}\]

Zener power dissipation:

\[P_Z = U_Z \times I_{Z,max}\]

Disadvantages

Poor efficiency (30-70%)
High power dissipation with large voltage differences
Temperature dependence of Zener voltage
Poor load regulation with large current variations
Limited output currents

Advantages

Simple design (only 2 components)
Low cost
Good voltage regulation
Fast response to load changes
Proven technology

Typical Applications

Reference voltages: ADC, DAC, OpAmp
Simple power supplies: Battery replacement
Overvoltage protection: Parallel to load
Voltage limiting: Signal processing
Bias voltages: Amplifier circuits

Zener Diode Characteristics

Voltage Range	Temperature Coefficient	Application	Special Features
2.7V - 4.7V	Negative (~-2mV/°C)	Low voltage regulation	Temperature compensation needed
5.1V - 6.8V	Minimal (~0mV/°C)	Reference voltages	Best temperature stability
7.5V - 22V	Positive (+2mV/°C)	High voltage regulation	Higher power types available

Design Considerations and Alternatives

When to use Zener diode regulator?

✓ Simple, cost-effective solution

✓ Constant or little varying load

✓ Output currents < 100mA

✓ Voltage difference < 10V

✓ Low efficiency requirements

Better Alternatives

LDO regulator: Higher efficiency, better regulation

Switching regulator: High efficiency (80-95%)

Reference ICs: Highest precision (LM4040)

Voltage divider: Only for high-impedance loads

Symbol Legend

U_in	Input voltage [V]
U_Z	Zener voltage (output voltage) [V]
I_L	Load current [A]
I_Z	Zener current [A]
I_Z,min	Minimum Zener current for stability [A]
I_total	Total current through series resistor [A]
R_s	Series resistor [Ω]
P_Rs	Power dissipation in series resistor [W]
P_Z	Power dissipation in Zener diode [W]

Is this page helpful?

Thank you for your feedback!

Sorry about that
How can we improve it?

Other electronics functions

LED resistor • Zener diode resistor (variable) • Zener diode resistor (fix) • Half-Wave rectification • Half-Wave rectification with capacitor • Full-Wave rectification • Full-Wave rectification with capacitor •